Object-oriented Design 1

1. Object-oriented Design 1

2. Objectives • To introduce an object-oriented design process for developing OO software. • To develop a case study (based on a weather monitoring system) that illustrates some of the models developed as part of an OO process. • To illustrate how an OO approach can lead to systems that can evolve in response to new requirements.

3. Object-oriented development • Object-oriented analysis, design and programming are related but distinct. • OOA is concerned with developing an object model of the application domain. • OOD is concerned with developing an object-oriented system model to implement requirements. • OOP is concerned with realising an OOD using an OO programming language such as Java, C++ or C#.

4. Characteristics of OOD • Objects are abstractions of real-world or system entities and manage themselves. • Objects are independent and encapsulate state and representation information. • System functionality is expressed in terms of object services. • Shared data areas are eliminated. Objects communicate by message passing. • Objects may be distributed and may execute sequentially or in parallel.

5. Object oriented modelling • An inherent part of object-oriented development is to develop UML models to represent the system. • Structural and behavioural UML models have already been introduced in previous lectures on system modelling. • Diagram types used here are from UML 1 rather than UML 2 but differences are minimal.

6. An object-oriented design process • Structured design processes involve developing a number of different system models. • They require a lot of effort for development and maintenance of these models and, for small systems, this may not be cost-effective. • However, for large systems developed by different groups design models are an essential communication mechanism.

7. Process stages • The principal activities in any OO design process include: • Context: Define the context and modes of use of the system; • Architecture: Design the system architecture; • Objects: Identify the principal system objects; • Models: Develop design models; • Interfaces: Specify object interfaces.

8. Weather system description A weather mapping system is required to generate weather maps on a regular basis using data collected from remote, unattended weather stations and other data sources such as weather observers, balloons and satellites. Weather stations transmit their data to the area computer in response to a request from that machine. The area computer system validates the collected data and integrates it with the data from different sources. The integrated data is archived and, using data from this archive and a digitised map database a set of local weather maps is created. Maps may be printed for distribution on a special-purpose map printer or may be displayed in a number of different formats.

9. Context • Develop an understanding of the relationships between the software being designed and its external environment • System context • A static model that describes other systems in the environment. Use a subsystem model to show other systems. Following slide shows the systems around the weather station system. • Model of system use • A dynamic model that describes how the system interacts with its environment. Use use-cases to show interactions

10. Layered architecture

11. Subsystems in the weather mapping system

12. Use-case models • Use-case models are used to represent each interaction with the system. • A use-case model shows the system features as ellipses and the interacting entity as a stick figure.

13. Use-cases for the weather station

14. Use-case description

15. Architecture • Once interactions between the system and its environment have been understood, you use this information for designing the system architecture. • A layered architecture as discussed in Chapter 11 is appropriate for the weather station • Interface layer for handling communications; • Data collection layer for managing instruments; • Instruments layer for collecting data. • There should normally be no more than 7 entities in an architectural model.

16. Weather station architecture

17. Objects • Identifying objects (or object classes) is the most difficult part of object oriented design. • There is no 'magic formula' for object identification. It relies on the skill, experience and domain knowledge of system designers. • Object identification is an iterative process. You are unlikely to get it right first time.

18. Approaches to identification • Use a grammatical approach based on a natural language description of the system (used in Hood OOD method). • Base the identification on tangible things in the application domain. • Use a behavioural approach and identify objects based on what participates in what behaviour. • Use a scenario-based analysis. The objects, attributes and methods in each scenario are identified.

19. Weather station description A weather station is a package of software controlled instruments which collects data, performs some data processing and transmits this data for further processing. The instruments include air and ground thermometers, an anemometer, a wind vane, a barometer and a rain gauge. Data is collected periodically. When a command is issued to transmit the weather data, the weather station processes and summarises the collected data. The summarised data is transmitted to the mapping computer when a request is received.

20. Weather station object classes • Ground thermometer, Anemometer, Barometer • Application domain objects that are ‘hardware’ objects related to the instruments in the system. • Weather station • The basic interface of the weather station to its environment. It therefore reflects the interactions identified in the use-case model. • Weather data • Encapsulates the summarised data from the instruments.

21. Key points • Object-oriented development involves adopting an OO approach at all stages from specification through to programming • OO design involves designing the system using objects as the fundamental abstraction and representing the system as an associated set of models in the UML • The OO design process involves several stages - discussed here were Context, Architecture and Objects.